Generally, a chassis dynamometer is accommodated in a pit. However, only rollers of the chassis dynamometer which are used for setting wheels of an automobile thereon are arranged to be partially exposed to an outside beyond a floor surface through an upper aperture of the pit. The upper aperture of the pit is normally covered with a pit cover for safety. Conventionally, a folding-type pit cover open/close device as disclosed in Patent Literature 1 and a sliding-type pit cover open/close device as disclosed in Patent Literature 2 are known.    Patent Literature 1: Japanese Patent Application Unexamined Publication No. 2002-116118    Patent Literature 2: Japanese Patent No. 3737910